Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sat
Alpine
4 · High
Treeline
3 · Considerable
Below
3 · Considerable
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Out of date — this forecast expired 5 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Over 2' feet of snow and recent strong wind are pushing a weak snowpack to its breaking point. Very dangerous avalanche conditions exist–particularly in windier terrain at upper elevations. You could trigger avalanches at any elevation today. Slides can be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. Some of these slides will be large enough to snap trees and involve the entirety of the season's snowpack.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2–3

    Where old October snow exists, the snowpack is teetering on the brink of failure. We've just added nearly 2" SWE to a slab that's now 4' thick and more snow is in the forecast. Here are some basics:

    • We have a known persistent weak layer. This layer is especially bad in shady (NW-N-NE-E facing) upper and middle elevation terrain.
    • SAC staff and public observers saw several large, powerful persistent slab avalanches (photo, photo) that released naturally Monday night or early Tuesday.
    • While evaluating conditions on Tuesday, Sun Valley Heli Ski staff remotely triggered a persistent slab slide when the helicopter landed on a flatter slope about 1/4 mile away (photo).

    The smallest of these persistent slabs will be large enough to bury you. On the upper end, they'll wrap hundreds of feet across the slope and break multiple feet deep, snapping trees as they go. Today is not the day to try to thread the needle and outsmart these slides with micro-scale route finding. Triggering one of these slabs from flatter terrain–below, above, and to the sides of steep slopes is a real possibility. When I look at this list, I can't imagine any other advice than to avoid traveling on or beneath steep slopes.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    The Banner Summit Zone picked up 11-23" (0.7-1.4" SWE) in the last 24hrs. This brings our storm totals up to 26-32" (1.6-1.9" SWE) since Wednesday morning. Given the periods of rapid snowfall and the variety of old snow surfaces, sensitive slabs may be found on many aspects and at all elevations. Slabs will be thicker, stiffer, and more dangerous as you climb in elevation where it's been windier and more snow has fallen.

    For days we've been finding safer conditions on low and middle elevation slopes that are more south-facing and lack widespread weak October snow near the ground (see Problem 1). Ongoing storms complicate matters as we build slabs on all aspects and elevations. This problem may worsen in the coming days as heavy snowfall returns to the forecast. 

Avalanche Discussion

FAR FROM OVER

When it comes to predicting avalanche conditions, you can never know where you're headed if you don't know where you've been. On an avalanche front, it's been an action-packed start to the winter beginning on December 11th with a week-long storm event that brought 3-5'+ (3-4" SWE) to the majority of the forecast area. Dozens of large avalanches were reported and many more went undetected as the snow just keep on coming. The vast majority of these slides occurred on N-NE-E-facing slopes (visual) and involved weak October snow near the ground.

Many of the slopes that didn't slide were left teetering on the brink of failure. A relatively small (but windy) storm on the 19th and 20th was enough to tip the scales in the southern Sawtooths and western Smokys. Numerous natural avalanches (photo, photo) were observed as skies cleared on the 21st. This small "test storm" is perhaps the most compelling clue in foreshadowing the effects of the larger storm knocking at our doorstep.

We'll see a break in the action today before the parade of precipitation events continues through the weekend. I'd encourage you to step back and look at the whole picture. In its simplest form, the avalanche danger will increase in elevation. This is due to increased snowfall, more wind-loading, and a more widely distributed set of weak layers as you climb in elevation.

Keep in mind, however, that there are a lot of question marks surrounding how this new slab of snow will bond to the old snow surface. Warmer temperatures and dense snowfall yesterday brought an increased chance of triggering a slab avalanche at lower elevations. It's not a stretch to imagine triggering a small 18" slab avalanche by snowshoeing too close to a steep slope or by sidehilling a roadcut off the valley floor. Today I'll happily use this uncertainty to motivate my choice to avoid avalanche terrain (including areas below steep slopes) until these storms are well in the rearview mirror. 

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